Factors affecting zero point of charge (zpc) of variable charge soils

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Abstract
The factors which determine or exert an influence on the zero point of charge (ZPC) of soils were examined, using 26 variable charge soils from Thailand and Japan. ZPC was mainly correlated with amorphous A1 for the Japanese volcanic ash soils, and with crystalline Fe oxides for the Thai Oxisols and Ultisols. Besides, significantly high correlations were found between the ZPC and the specific surface area, and phosphate absorption coefficient. Sample soils were categorized into three groups on the basis of the ZPC value, namely, Group 1 with a ZPC value higher than 5.5, Group 2 with a ZPC value ranging from 4.5 to 5.5, and Group 3 with a ZPC value lower than 4.5. Amorphous A1 oxides for Groups 1 and 2 and crystalline Fe oxides for Group 3 were the prominent factors relevant to ZPC. Among other physico-chemical properties, the total carbon content was closely associated with the ZPC variations. No definite sequence of ZPC within a pedon was observed for the volcanic ash soils, reflecting the difference in parent materials and genetic processes. On the other hand, for the strongly weathered soils, lower values of ZPC were recorded in the surface layer than in the subsurface layers, reflecting the accumulation of organic matter in the former and iron oxides in the latter.